In conventional heating furnaces, a variety of workpieces are continuously heat treated by sequentially transferring the workpieces. As such a heating furnace, the walking-beam type heating furnace is widely used in which the workpieces retained on the fixed beams are heat treated as sequentially transferred by the walking beams performing walking motion.
It is a general practice in such a walking-beam type heating furnace that the fed workpieces are heated in a heating zone of the inlet area while the heated workpieces are wholly heated to the uniform temperature and retained in position in a soaking zone of the outlet area.
In this walking-beam type heating furnace, the elongate workpieces, placed in the direction crossing the transfer direction, are retained on the fixed beams and heat treated as transferred by the walking beams. For this operation, the fixed beams and the walking beams are each formed with corrugated concaves having the same groove depth and pitch in the transfer direction. The walking beams are driven into the walking motion so as to sequentially transfer the workpieces retained in the concaves of the fixed beams to the downstream concaves of the fixed beams in the transfer direction, as allowing the workpieces to be heated.
However, when the elongate workpieces are heat treated as transferred by the walking beams, heating temperature difference occurs in the heating zone of the inlet area between place close to a burning area and place far away from the burning area, so that the elongate workpieces are not uniformly heated, bulgingly curving in the direction crossing the longitudinal direction thereof. Specifically, when the workpiece is fed into the heating furnace, a door is opened to allow the outside air to flow into the furnace. Therefore, the temperature in the heating zone is lower at an upstream side (inlet side) in the transfer direction of the workpiece than at a downstream side (where the burning area exists). Accordingly, a downstream side of the workpiece in the transfer direction of the workpiece is raised to a higher temperature than an upstream side thereof and elongates more by heat. This results in curving of the workpiece. Particularly, in a case where the elongate workpiece is a tubular member as exemplified by steel pipe or the like which has small diameter and thickness, the curving of workpiece due to the heating temperature difference increases. This leads to a problem that the workpieces in the individual concaves, colliding against or overlapping with each other, cannot be properly transferred or adequately heat treated.
In this connection, Patent Document 1 proposes that in at least either one of the fixed beam and the walking beam, a carrying surface on which a workpiece having a circular section is placed is partially formed with a slope to allow the workpiece to roll down by its own weight and rotate through a predetermined angle and that the concaves on the fixed beam have a different pitch from a drive pitch of the walking beam. It is thus ensured that the workpiece is allowed to roll on this slope so as to be uniformly heated while the workpiece is heat treated as transferred by the walking beam. Thus, the workpiece is prevented from bulgingly curving in the direction crossing the longitudinal direction thereof.
However, even though the carrying surface on which the workpiece having the circular section is placed is partially formed with the slope to allow the workpiece to roll down by its own weight and rotate through a predetermined angle, if the workpiece becomes significantly curved shortly after fed into the furnace, the workpiece is incapable of smooth rolling. In the heating zone of the inlet area, the workpiece is not assuredly prevented from curving. There still exists the problem that the workpieces, colliding against or overlapping with each other, cannot be properly transferred and adequately heat treated. There are other problems that the above slope formed on the carrying surface must be increased in length in order to prevent the curving of the workpiece by allowing the workpiece to roll, and that the heating furnace must be increased in length in order to achieve uniform heat treatment of the workpieces by adequately heating the workpieces in the heating furnace.
Further, Patent Document 2 proposes that when the workpieces composed of steel pipe are heat treated as transferred on a roller conveyor, a curving condition of the steel pipe pieces being transferred is sensed to exclude a significantly curved steel pipe piece.
However, the arrangement of Patent Document 2 cannot prevent the curved workpieces from colliding against or overlapping with each other when the curved workpieces are transferred. In addition, a large number of workpieces are wasted because the significantly curved steel pipe pieces are excluded.